1. Field of the Invention
The invention relates to scientific measurement devices, more particularly to a method and apparatus for measuring the surface tension of a liquid or the interfacial tension between two dissimilar liquids, by measuring the reduction in weight of the sample liquid as a probe rigidly position to prevent lateral movement is vertically pulled away from the sample rather than by directly measuring the force required to pull a probe away from the sample.
2. Description of the Prior Art
Surface tension is defined as a property of liquids in which the boundary tends to contract to the smallest possible area because of unequal molecular cohesive forces near the boundary. It is measured by the force required to break an object or probe free of the boundary. This boundary may be the exposed surface of the liquid to the air or it may be between two immiscible liquids; when referring to the surface tension of the boundary between the liquid under investigation and another liquid, we use the term "interfacial tension", but for purposes of this invention, the term surface tension is assumed to include both. Surface tension is manifested by a phenomenon known in physics as the meniscus of the liquid, which is the concavity or convexity of the liquid's surface as it interacts with the probe. As a result of capillarity, the liquid's surface is concave against the probe when the probe strongly attracts the liquid, as with water, and it is convex when the liquid is more strongly attracted to itself, as with mercury.
Many methods have been proposed for measuring surface tension of a liquid and interfacial tension between two dissimilar liquids. One technique is to use maximum bubble pressure methods where a gas is passed through an orifice tube into the test liquid, forming a succession of bubbles. Since the inflation pressure inside each bubble is at a maximum when the bubble achieves minimum radius of curvature as the bubble assumes hemispherical shape at the orifice, the maximum bubble pressure is directly related to the surface tension of the liquid. However, these methods require are costly in terms of the time required to perform valid measurements. Other so called static equilibrium techniques rely on measuring the vertical force required to remove a free hanging probe (such as a plate, rod, ring, or other solid object) from a liquid as the surface tension force of the liquid resists the removal of the probe. Such measurements are performed fairly rapidly, but they methods require a minimum sample size to be effective.
All of these static equilibrium methods of measuring surface tension require that a probe be suspended above and in the same plane as the liquid(s). The probe is attached to a sensitive balance, such as a torsion balance or a microbalance, mounted above the liquid(s), so that the force exerted by the surface tension of the liquid can be measured while the probe is pulled upward or pushed downward through the plane surface of the liquid(s). The maximum required force is found prior to the probe breaking through its meniscus with the liquid, and surface tension can be calculated from this value of force.
These static equilibrium methods exhibit several disadvantages. One disadvantage is that they require expensive and sensitive analytical balancing mechanisms to measure overhead the force exerted on a suspended probe. These mechanisms are expensive, sensitive, and awkward to work with. Recently, top-loading analytical balances have been developed that automatically maintain the weighted object at the same height. These devices are more convenient to use and are less expensive than the more traditional methods of weighing probes. They use modem technologies such as piezoelectric devices or load cells to measure force without movement of the object being measured.
A second disadvantage is the delicacy of the probe used for measurement. A standard configuration for a probe is commonly called the "du Nouy ring", which consists of a wire ring of known dimensions composed of platinum-iridium alloy. The wire ring must be carefully calibrated so that the thickness of the wire forming the ring and the diameter of the ring are known precisely. The ring must be handled very carefully so as not to deform the circularity of the ring and thus adversely affect its calibration. The wire ring must be handled so as to prevent oily films from being deposited on the ring by the fingers; this affects the manner in which the ring interacts with the liquid whose surface tension is being measured. And finally, the wire ring must be constructed from special alloys, preferably platinum-iridium, which will permit the ring to be cleaned by passing it through a flame, thus burning off unwanted oils without deforming the ring through thermal stress. The same considerations apply more or less to other probe configurations, such as the Wilhelmy plate configuration. If any of the above-mentioned actions is performed carelessly, then the measurement becomes suspect and thus invalid.
A third disadvantage of the traditional measurement method is that it is ineffective for measuring relatively small liquid samples. Capillary action tends to attract the probe to the sides of the container. If the sample size is large enough whereby the probe does not directly interact with the sides of the container, then an adjustment factor must be applied to account for the volume of liquid in the meniscus between the sides of the probe and the interior walls of the container. This adjustment is accomplished by mathematical integration and is given in two articles by D. N. Furlong and S. Hartland, Journal of the Chemical Society Faraday I, vol 76, p. 457 (1980) and vol 76, p. 467 (1980). A free hanging probe is attracted to the walls of the container holding the liquid sample. For small containers and sample sizes, this capillary action of the walls of the become so severe that it invalidates the measurements since a free-hanging probe cannot be kept from contact with the container walls. This traditional method of measuring surface tension is thus not suitable for small sample sizes on the order of 0.4 mL or less.
A fourth disadvantage of traditional methods, especially for volatile liquids, is that evaporation of the liquid during the measurement process can adversely skew the measurement. Evaporation can be reduced by use of small openings through which the probe is inserted or by using small sample sizes.
Thus, traditional methods of measuring surface tension have the following disadvantages:
a. expensive balances mounted above the liquid sample are required to measure the force exerted on the probe; PA1 b. the probe must be very carefully fabricated and handled; PA1 c. surface tension can only be measured for relatively large sample sizes; and PA1 d. evaporation can adversely affect the measurement.
What is needed is a device for measuring surface tension which avoids the problems of excessive delicacy, expense, and sample size.